So far this year, four horses have been diagnosed with West Nile virus in California: in Sacramento, San Joaquin, and Madera Counties. Each of the four horses was either unvaccinated or not vaccinated completely.

State officials have renewed calls for all horse owners to vaccinate, warning that outbreaks of the virus are still a risk for horses.

"Horse owners should contact their veterinarians as soon as possible to ensure vaccination status is current," said California State Veterinarian Dr. Richard Breitmeyer.

Symptoms of West Nile virus include stumbling, staggering weakness, muscle twitching, and inability to stand.